<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果:<span></span></div>

<script>
    function f() {
        //得到文本框和span
        let i =document.querySelector("#i1");
        let s = document.querySelector("span");
        //"5"*"5"   5*5
        //"a"*"a"    NaN*NaN  = NaN   不是一个数   NaN与任何相乘都等于NaN
        /*isNaN()判断变量是否是NaN*/
        if (isNaN(i.value)){
            s.innerText="输入错误！";
            return;
        }
        //s.innerText=i.value*i.value+"";
        //当两个字符串进行-*/运算时会自动转成数值，进行+运算时是字符串拼接
        //s.innerText =i.value*1+i.value*1;
        //parseFloat()将字符串转成整数或小数
        s.innerText =parseFloat(i.value)+parseFloat(i.value);
    }
</script>
</body>
</html>